Method of driving liquid crystal display device

ABSTRACT

A method of driving a liquid crystal display device including a liquid crystal layer provided with electrodes on its two opposite surfaces, comprising the steps of applying a first signal for designating the electrode position and a voltage signal having a higher frequency to the electrodes on one surface, and applying and input signal and the signal of said higher frequency to the electrodes on the other surface. The signals are controlled by another signal of different frequency to drive the display device with an ac voltage. Temperature compensation can be performed by varying the magnitude or the frequency of the voltage signal according to the temperature level.

United States Patent [191 Fukai et al.

[451 Nov. 18, 1975 METHOD OF DRIVING LIQUID CRYSTAL 3.740.717 6/1973 Huener et al 350/160 LC DI P D S LAY EVICE OTHER PUBLICATIONS [75] Inventors: Masakazu Fukai, Nishinomiya;

S ii hi Nagata Sakai; Komei Asai, IBM Tech. D1s cl. Bull Vol. 15, No. 6, 11/72, p. 1811 Hi k Katsuji H Uji all f Thermally Activated Lzqmd Crystal Display; Powers. Japan 73 Assignee: Matsushita Electric Industrial (30., Primary hammer-Marsha" Curtis,

Ltd Osaka Japan Attorney, Agent, or Fzrm-Stevens, Davis, Miller &

Mosher [22] Filed: Apr. 4, 1973 [21] Appl. No.: 347,631 [57] ABSTRACT A method of driving a liquid crystal display device in- [30] Forelgn Apphcatmn Prlomy Data cluding a liquid crystal layer provided with electrodes Apr. 6, 1972 Japan 47-34984 on i two Opposite Surfaces comprising the steps f June 16, Japan a first signal for designating the electrode p0- June 29, 1972 Japan 47-65783 i i and a voltage Signal having a higher frequency to the electrodes on one surface, and applying and [52] 340/324 M; 219/216; 340/336; input signal and the signal of said higher frequency to 2 350/160 LC the electrodes on the other surface. The signals are [51] Int. Cl. G09F 9/32 controlled by another Signal of different frequency to [58] M Search 340/324 166 336; drive the display device with an ac voltage. Tempera- 178/30; 350/160 LC; 219/216 ture compensation can be performed by varying the magnitude or the frequency of the voltage signal ac- [56] References Clted cording to the temperature level.

UNITED STATES PATENTS 3,725,898 4/1973 Canton 178/30 4 Clams 36 Draw'ng Fgures DRIVE COM- l MANDER 1 TEMPERATlRE VAR'ABI-E t DETECTOR IQE DRIVE 44 SOU L 42 COMMANDER LOGIC 46 DRIVER /43 CONSTANT VOLTAGE SOURCE Patsnt Nov. 18, 1975 Sheet10f18 3,921,162

US. Patent Nov. 18, 197 5 Sheet 2 of 18 3,921,162

US. Patent Nov. 18, 1975 Sheet3of 18 3,921,162

m w u US Patent Nov. 18,1975 Sheet40f 18 3,921,162

UQSQ Patent Nov. 18, 1975 LIGHT SCATTERING THRESHOLD VOLTAGE( V) Sheet 5 of 18 3,921,162

DISPLAY U.S. Patent Nov. 18,1975 Sheet60f18 3,921,162

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F l G 8 O CONTROLLER M 54 f 52 /5O 53 OUTPUT w AMP. COMPARATOR VOLTAGE DETECTOR fi D. C 55 DIFFERENCE 5; SOURCE VOLTAGE U.S. Patent Nov. 18,1975 Sheet80f 18 3,921,162

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METHOD OF DRIVING LIQUID CRYSTAL DISPLAY DEVICE This invention relates to a method of driving a liquid crystal display device.

A liquid crystal display device basically consists of a liquid crystal layer, a pair of electrodes for applying a voltage to the liquid crystal, and a voltage source connected between said pair of electrodes. The application of a voltage to a liquid crystal changes the state of the liquid crystal to which the voltage has been applied and enables a display thereby. Nematic and choresteric liquid crystals are used as the liquid crystal. The following description is based, however, on the general nematic liquid crystal.

When a voltage is applied to a nematic liquid crystal layer above a certain threshold voltage, it generates a turbulent state in the portion to which the voltage has been applied. This state grows as the voltage increased and reaches the so-called dynamic scattering state in which light scattering becomes extremely large. Such a phenomenon can be used to display a pattern of letters and/or figures. Namely, a pattern display can be achieved by disposing the the x and y electrodes on the two surfaces of a liquid crystal layer to form a display of the matrix structure and to produce a spatial scattering state in the liquid crystal.

In such a display, however, when a voltage is applied across selected x and y electrodes to excite the crossed point, adjacent crossed portions of the electrodes may also be driven into the scattering state; the socalled cross-talk phenomenon may occur. This is due to the appearance of a voltage in the neighboring electrodes through the static capacitance between the electrodes and the electrical resistivity (conductivity) of the liquid crystal. When such an induced voltage exceeds the threshold voltage of the liquid crystal, it generates a dynamic scattering state.

The cross-talk phenomenon makes the outline of the displayed pattern ambiguous and may cause a viewer to erroneously interpret the display. Thus, various methods have been proposed to substantially suppress the generation of the cross-talk phenomenon. For example, the electrodes of the x axis direction are maintained at a certain voltage and those of the y axis direction have a high frequency voltage of one polarity applied thereto. When an input signal is supplied, a voltage is applied to the designated x electrode to generate an electric field of the same polarity as said high frequency voltage and the designated y electrode is separated from the application of said high frequency voltage. By this method, a high frequency voltage is superposed on the cross-talk voltage and the composite voltage prevents the generation of the dynamic scattering state. Here, the magnitude and the frequency of the high frequency voltage is so selected that the composite voltage does not generate the dynamic scattering state. In practice, from the limitation of the circuit arrangement, the magnitude of the high frequency voltage becomes equal to the dc voltage applied to the designated pair of electrodes. Therefore, the magnitude and the frequency of the voltage applied between the electrodes should be suitably selected to drive only predetermined portions into the light scattering state. Further, this voltage application is essentially a dc drive and is accompanied with the inconvenience of short operative service life.

An object of this invention is to provide a method of driving a liquid crystal display device based essentially on an ac drive, and which is capable of prolonging the activation lifetime of the liquid crystal display several times beyond that for the dc drive without generating the cross-talk phenomenon.

Another object of this invention is to provide a method of driving a liquid crystal display device in which the driving voltage for the liquid crystal layer is controlled by the temperature of the crystal or by the ambient temperature.

A further object of this invention is to provide a method of driving a liquid crystal display device in which the repetition time of the voltage signal applied to the liquid crystal layer is controlled by the temperature of the liquid crystal or by the ambient temperature.

According to an embodiment of this invention there is provided a method of driving a liquid crystal display of the matrix type including a liquid crystal layer and two groups of electrodes disposed on the respective surfaces of the liquid crystal layer comprising the simultaneous steps of applying a first signal designating the position of the electrodes and a second voltage signal having a higher frequency than that of said first signal through the gate control by a third signal voltage having a different frequency than that of the first signal to one group of electrodes, and applying to a designated electrode of the other group of electrodes an input signal for producing light scattering in a selected portion of the liquid crystal and said second signal voltage through the gate control by said third signal voltage, thereby periodically inverting the polarity of the voltage applied to the electrodes across said selected portion of the liquid crystal.

According to the above method of driving a liquid crystal display device, the polarity of the voltage applied to the display portion of a liquid crystal display of a matrix structure for producing light scattering can be reversed in time. Thus, the liquid crystal layer has applied thereto an alternating voltage of a certain period. Therefore, the activation life time of the liquid crystal in the above method becomes longer than that obtained when a voltage of one is applied. This alternating voltage can be formed of signal components of one polarity. This greatly relaxes the restrictions in the circuit design of the driving unit. The effects due to the crosstalk phenomenon can be suppressed by supplying a voltage of high repetitive frequency. Further, according to the above method, since a matrix liquid crystal display is driven by a voltage which is controlled by temperature, the usable temperature range is greatly widened in comparison to that for a constant voltage drive. Accordingly, even when the temperature of the display is high, drive with a low pulse voltage works well and hence the lifetime of the liquid crystal can be extended.

Further objects, features and advantages of the present invention will become apparent in the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of a structure embodying the present method of driving a liquid crystal display device;

FIGS. 2 and 3 show signal waveforms at the various portions of the circuit of FIG. 1 in operation;

FIG. 4 shows operational characteristic curves of the display of FIG. 1; 

1. A method of driving a liquid crystal display device having a matrix structure including a nematic crystal layer and two groups of electrodes disposed on the respective surfaces of said liquid crystal layer, each of the electrodes in said two groups being driven by an associated controller having an inpput and a gating circuit, said method comprising the steps of applying first and second pulse driving voltages to the inputs of first and second selected controllers, each selected controller being associated with an electrode of one of said two groups to designate a selected electrode position, applying a first a.c. signal voltage having a frequency higher than that of said first or second pulse driving voltages to the inputs of the controllers associated with said two groups of electrodes, applying a second a.c. signal voltage having a frequency different from that of said first or second pulse driving voltages to the gating circuits of the controllers associated with said two groups of electrodes, said second a.c. signal controlling application of said pulse driving voltages and first a.c. signal to said electrodes, measuring the temperature adjacent said nematic liquid crystal, and changing the magnitudes of the voltages applied to all of the electrodes of said two groups to follow changes in the temperature of said liquid crystal layer, said changes in the magnitudes of the voltages applied to said electrodes causing dynamic scattering in said liquid crystal at the changed temperatures thereof at said selected electrode position and suppressing dynamic scattering at the other electrode positions, whereby the temperature range wherein dynamic scattering is produced is widened.
 2. The method of driving a liquid crystal as defined by claim 1 which further includes the steps of inverting said first and second a.c. signal voltages before applying them to the inputs and gating circuits respectively of said controllers.
 3. A method of driving a liquid crystal display device having a matrix structure including a nematic crystal layer and two groups of electrodes disposed on the respective surfaces of said liquid crystal layer, each of the electrodes in said two groups being driven by an associated controller having an input and a gating circuit, said method comprising the steps of applying first and second pulse driving voltages to the inputs of first and second selected controllers, each selected controller being associated with an electrode of one of said two groups to designate a selected electrode position, applying a first a.c. signal voltage having a frequency higher than that of said first or second pulse driving voltages to the inputs of the controllers associated with said two groups of electrodes, applying a second a.c. signal voltage having a frequency different from that of said first or second pulse driving voltages to the gating circuits of the controllers aSsociated with said two groups of electrodes, said second a.c. signal controlling application of said pulse driving voltages and first a.c. signal to said electrodes, measuring the temperature adjacent said nematic liquid crystal, and modifying the frequency of the voltages applied to the electrodes of said two groups in accordance with the measured temperature of said liquid crystal layer, whereby dynamic scattering is produced in said liquid crystal at said selected electrode position and dynamic scattering is suppressed at the other electrode positions.
 4. The method of driving a liquid crystal as defined by claim 3 which further includes the steps of inverting said first and second a.c. signal voltages before applying them to the inputs and gating circuits respectively of said controllers. 